Digital radiography is increasingly accepted as an alternative to film-based imaging technologies that rely on photosensitive film layers to capture radiation exposure and thus to produce and store an image of a subject's internal physical features. With digital radiography, the radiation image exposures captured on radiation-sensitive layers are converted, pixel by pixel, to electronic image data which are then stored in memory circuitry for subsequent read-out and display on suitable electronic image display devices.
The perspective view of FIG. 1 shows a partial cutaway view of a small edge portion of a DR panel 30 of the indirect type. A scintillator screen 12 responds to incident x-ray radiation by generating visible light that is, in turn, detected by a flat panel detector (FPD) 32. Detector 32 has a two-dimensional array having millions of radiation sensitive pixels 24 that are arranged in a matrix of rows and columns and are connected to readout element 25. Readout element 25 is generally termed an ASIC (Application-Specific Integrated Circuit) or ASIC chip. As shown at enlarged section E, each pixel 24 has one or more photosensors 22, such as a photo-responsive diode or other light-sensitive component, and an associated switch element 26 of some type, such as one or more thin film transistors, or TFTs. To read out image information from the panel, each row of pixels 24 is selected sequentially and the corresponding pixel on each column is connected in its turn to a charge amplifier (not shown). The outputs of the charge amplifiers from each column are then applied to ASIC chips and related circuitry that generate digitized image data that then can be stored and suitably image-processed as needed for subsequent storage and display.
Each row of pixels 24 extends several hundred pixels in length. In the readout sequence, all pixels in the row are generally read at the same time. To do this, signals from each pixel in that row are simultaneously switched, in a multiplex fashion, to a set of multiple ASIC readout elements 25, each ASIC readout element 25 reading multiple pixels within that row. In one exemplary embodiment, for a DR panel with a row length of 2560 pixels, a set of 20 ASIC chips is provided for reading pixels in a row, each ASIC connecting to 128 pixels at a time.
This type of multiplexed readout sequencing arrangement has been widely adapted for digital image sensors in general and has shown itself to be practical. By grouping and sharing larger and more complex readout components so that successive rows of pixel sensors can obtain the image signal and then be read out by the same signal acquisition circuitry, this readout approach helps to optimize the usable image-forming area of FPD 32. However, this multiplexed readout arrangement is not without its problems. Component packaging densities, signal crosstalk, switching noise, so-called “white noise” from sensor circuitry, and other unwanted conditions result in some inherent level of signal noise that can interfere with image content. Noise from any of these sources can be perceptible in the final image, depending on the type of noise and on the relative sensitivity of different components to noise.
Some types of noise can be corrected for, or at least reduced in effect, using calibration or other conventional practices. Noise resulting from electromagnetic interference (EMI), however, is one type of noise that is not so easily handled and can be particularly troublesome. Artifacts that result from EMI-induced noise can occur due to electromagnetic (EM) fields generated by nearby equipment, such as power transformers and switching circuits. The EM fields that are generated by such equipment can have a field strength that varies both temporally and spatially. Not only can it be difficult to accurately predict the behavior of EM fields from neighboring equipment, which varies according to the relative placement of the DR receiver panel, but these same EM fields change with time and can affect voltage signals in the DR receiver panel during or after image exposure as well as during image data readout. EMI variations are not synchronized with the timing of electronic component operation at any point during image capture.
Artifacts from EMI-induced noise may not be perceptible along an entire row of pixels, but may affect only a portion of a row or column. Techniques that identify and compensate for row noise, therefore, may have limited value for addressing some types of EMI-related noise problems. Thus, solutions such at those presented by Maolinbay et al. in the article entitled “Additive noise properties of active matrix flat-panel imagers” in Medical Physics, August 2000, while they may help to minimize some types of one-dimensional image problems in DR detectors, prove disappointing for correcting two-dimensional artifacts from EMI-induced noise.
Conventional solutions for reducing EMI depend largely on the relative frequency of the EM signal. Compensation for magnetic fields can include enhanced shielding of the DR receiver panel as well as of nearby equipment and connectors. This solution can have some value for helping to reduce EMI affects. However, it can be impractical to provide sufficient shielding for other equipment outside the DR receiver panel. Moreover, shielding of the panel itself can tend to reduce its sensitivity, requiring that the patient receive higher levels of radiation as a result.
Another type of solution, as described, for example, in U.S. Pat. No. 7,091,491 entitled “Method and Means for Reducing Electromagnetic Noise Induced in X-ray Detectors” to Kautzer et al. addresses the EMI problem by detecting the interfering signal directly using a network of field sensing conductors directly integrated into the detector circuitry. This detected signal (image plus EMI noise) is then used to condition the detected image signal in an attempt to reduce the EMI induced noise. However, such a solution is fairly complex, adding to fabrication expense and complexity and makes a number of assumptions about EMI that may not be true in every environment. Other solutions include filtering signal lines and applying feedback control to power supplies. These solutions, however, are system-dependent and can be impractical for retrofit applications.
U.S. Pat. No. 6,819,740 entitled “X-ray Diagnosis Apparatus Having a Flat Panel Detector for Detecting an X-ray Image” to Takahashi et al. describes an x-ray diagnosis system with signal processing for reducing what is described as ‘line artifact noise’. This is described as having a high frequency component which is stable in the row direction and varying in the column direction. As best understood by the inventors, the assumption is that the gate line direction for reading image data is along the image rows. A line-by-line vector is derived from the image array. These values are then used to correct the medical image array, line-by-line, using a value from this vector. This can be done, for example, by subtracting an offset value for each pixel in a line, based on the corresponding value from the error vector. This method presupposes a fixed direction of the artifact fluctuations. While such an assumption may prove workable for a specific system having tightly integrated image acquisition and processing hardware, however, it does not address the more general case of ‘open system’ applications. A drawback of this type of approach is that it works acceptably only when noise affects every pixel in a row in the same way. This approach would not work well in cases where noise affects the line or row of pixels differently, depending upon their relative position along the line.
Thus, although various solutions have been proposed for addressing the problem of EMI-induced noise with DR imaging, these various approaches do not deal successfully with the situation wherein noise effects from EMI or other sources may vary spatially and temporally not only between systems, but also for a DR receiver used as part of the same imaging system and where these effects on the image can be difficult to predict.